Fox, 27, is married to former "Beverly Hills, 90210" star Brian Austin Green, 40. They are already the parents of a 10-month-old son named Noah.
"I can confirm Megan is expecting her second child with her husband Brian," the actress' representative told the New York Post. "They are both very happy."
The baby's gender and anticipated due date were not disclosed.
Green also has an 11-year-old son from his prior relationship with actress Vanessa Marcil.
Fox is known for her work in the "Transformers" film franchise.